The trial is expected to be the first step in a legal process that will likely land the case in the U.S. Supreme Court.

A lesbian couple in Berkeley and a gay couple from Burbank filed the suit to overturn Prop. 8, the state's ban on same-sex marraige, claiming that the ban violates their federal constitution rights. The trial is set for January 11.

U.S. District Chief Judge Vaughn Walker had the option of throwing the case out altogether Wednesday, but refused to do it after listening to both sides Wednesday morning.

What the judge said in court could be seen as even a bigger potential win for gay rights advocates than his ruling.

The San Francisco-based judge said he wants proof that allowing gay couples to wed threatens traditional male-female unions.

At one point Walker grilled a pro-Proposition 8 lawyer who was claiming the law was legitimate because it fostered "naturally procreative relationships."

The judge demanded to hear how that goal would be undermined if same-sex marriages were legal.